1. Field of the Invention
The present invention relates to a signal transmission cable with a connector, which requires measures against EMI (electromagnetic interference). More particularly, the present invention relates to a signal transmission cable with a connector, which can respond to noise regulations prescribed by every country without impairing appearance and handleability by combining a closed magnetic path core embedded in a folded portion of a shielding layer of a cable, and a shielded cable having a magnetic powder compound layer. This technology is useful for various signal transmission cables used in, for example, computers game machines, office equipment, portable devices, medical equipment, in-vehicle equipment, machine tools, and the like.
2. Description of the Related Art
In recent years, along with the increased processing speed in electronic equipment, errors caused by electromagnetic interference noise have become a problem. Accordingly, as for a signal cable which transmits and receives several ten Mbps signals, in order to reduce unnecessary electromagnetic radiation attributable to a common mode current, there have been heretofore taken various measures as described below.
(1) Attachment of Low-pass Filter to Signal Line
A filter circuit including a capacitance, an inductance unit or a combination thereof is connected to an output terminal of a single signal transmission circuit.
(2) Attachment of Common Mode Choke to Signal Line
By attaching a common mode choke to an output terminal of a signal transmission circuit, balance of a signal is enhanced, and a common mode current is reduced.
(3) Use of Shielded Cable and Connector
A signal line is shielded by covering the signal line with a metal plate or a wire mesh.
(4) Attachment of Ferrite Core to Outside of Cable Insulating Coating
By attaching a ferrite core to outside of cable insulating coating, a common mode current flowing through a shielding layer of a cable is suppressed. The ferrite core is housed in a resin case, for example, which is divided into two parts and can be united in a snap-fit manner. The ferrite core is fitted from the outside of the cable.
(5) Use of Ferrite Cable
By interposing a ferrite compound layer (a layer having ferrite powder mixed in a resin material) between a shielding layer (braided shield) and insulating coating of a cable, a common mode current flowing through the shielding layer is suppressed.
However, when trying to reduce unnecessary electromagnetic radiation of a cable for a high-speed signal of over several hundred Mbps by use of the conventional technology as described above, the following problems were observed.
(1) Attachment of Low-pass Filter to Signal Line
In order to transmit and receive a signal at a transmission rate of several hundred Mbps, a rise time and a fall time of a digital waveform have to be set to several hundred picoseconds. In order to transmit and receive a signal without a bit error, it is required to ensure the 6 dB bandwidth of a transmission line to be up to several GHz. However, when a method of attaching a low-pass filter to a signal line is used to try to conform to regulations for unnecessary electromagnetic radiation prescribed by every country, it is required to set a cut-off frequency of the low-pass filter to several ten MHz. Accordingly, the 6 dB bandwidth of the transmission line, which is required for signal transmission, cannot be ensured.
(2) Attachment of Common Mode Choke to Signal Line
A common mode choke basically reduces only a common mode current, and does not affect a differential or a single signal. However, since an actual common mode choke has differences in resistance of a coil and a wire length, when the frequency reaches several ten MHz or more, the common mode choke starts to function as a low-pass filter for the differential or the single signal. Thus, a bit error is caused by received waveform rounding in signal transmission of several hundred Mbps or more.
(3) Use of Shielded Cable and Connector
In actual shielded connector and shielded cable, electrical connection on a contact surface between metal plates or between a metal plate and a braided shield is not perfect. Generally, the higher the frequency gets, the more the contact impedance between the metal plates or between the metal plate and the braided shield is increased. Accordingly, a shielding effect diminishes from around 800 MHz. Moreover, when a common mode current flows through a differential signal line in the shielded cable, the common mode current returns to a source of the signal via the braided shield of the shielded cable. Thus, unnecessary electromagnetic radiation is generated from the braided shield. Accordingly, only by use of the shielded cable and connector, a frequency band having a sufficient shielding effect to reduce unnecessary electromagnetic radiation of a signal having a transmission rate of several hundred Mbps is narrow. Consequently, a sufficient reduction effect cannot be obtained for the common mode current caused by unbalance of a differential signal.
(4) Attachment of Ferrite Core to Outside of Cable Insulating Coating
Since a ferrite core fitted to outside of insulating coating of a cable is large and heavy, flexibility becomes poor. Thus, not only does handling of the cable become difficult, but also the appearance thereof is impaired. Moreover, assembly and attachment costs of the core are increased. Furthermore, since magnetic permeability is lowered at a frequency as high as 800 MHz or more, a sufficient common mode current suppression effect cannot be obtained. A signal having a transmission rate of several hundred Mbps or more has an electrical energy of up to several GHz. Thus, an unnecessary electromagnetic radiation reduction effect at the frequency as high as 800 MHz or more is insufficient.
(5) Use of Ferrite Cable
A cable having a ferrite compound layer exerts a stable common mode current suppression effect at a frequency of 100 MHz or more. In addition, the cable has a smart appearance and good flexibility (bendability). However, the cable has hardly any common mode current suppression effect at a frequency of 100 MHz or less. Thus, a reduction effect cannot be obtained for a low-frequency common mode current of a signal having a transmission rate of several hundred Mbps or more.